Internal combustion engines may utilize an exhaust gas recirculation (EGR) system to re-circulate a controlled portion of exhaust gas generated by the engine into an intake manifold of the engine. Similarly, variable valve mechanisms in internal combustion engines may also be used to enhance engine performance by improving intake efficiency and decreasing exhaust emissions. Control systems have been provided which vary the EGR rate and/or valve timing according to one or more sensed conditions, such as engine temperature, air charge and engine speed, to thereby improve engine drivability and emissions.
One example approach for EGR control is shown by Cullen et al. in U.S. Pat. No. 5,515,833. In this example, the EGR rate in an engine is adjusted based on an engine speed, an air charge value and an ambient barometric pressure. Specifically, a base EGR rate is computed as a function of engine speed and prevalent air charge. Next, a maximum EGR rate is computed as a function of the ratio of the prevalent air charge to the peak air charge, further compensating for a given barometric pressure. The EGR rate is then gradually blended from an initial value of zero to the maximal EGR rate level over a predetermined period of time.
However, the inventors herein have recognized several issues with such an approach. As one example, a change in peak air charge may not necessarily correlate with changes in peak engine torque output. In other words, in Cullen et al., it was possible to compensate for the changes in peak engine output due to barometric pressure changes using the peak air charge. However, in directly injected flexible fueled vehicles capable of operating on a variety of fuels and fuel combinations, changes in peak air charge may not correlate with changes in peak torque. Specifically, peak torque of the engine may change at a given altitude due to changes in fuel composition, thus changing the charge cooling and/or octane effectiveness of the injected fuel. As such, the system may provide too much, or too little EGR for various operating conditions. This can result in degraded fuel economy when too little EGR is provided, and degraded drivability when too much EGR is provided.
Thus, in one example, the above issues may be addressed by a method of operating an engine in a vehicle, the method comprising: operating the engine with a variable fuel blend in a cylinder, where the variable fuel blend varies a peak achievable engine torque for a given operating condition and selectively operating an engine actuator that affects engine torque and engine fuel economy at the given operating condition. The method further comprises extending operation of the actuator to higher engine torques as a peak engine torque for the given operating condition increases.
In one example, the engine actuator operates an EGR schedule such that an EGR amount is gradually phased in as the torque approaches a predetermined threshold, such as a mid or low torque, and then the EGR amount is gradually phased out as the torque approaches the peak achievable torque for the given fuel blend. As such, a peak achievable torque may change responsive to changes in fuel composition. Accordingly, the engine actuator may phase in and phase out the EGR amount differently over the dynamically changing achievable torque range, responsive to the current fuel composition.
In another example, the engine actuator operates a late intake valve closing (LIVC) schedule such that an LIVC amount is gradually phased out as the torque approaches the peak achievable torque for the given fuel blend. As the fuel composition varies (e.g., due to refueling events), and consequently the operating torque range varies, the engine actuator may phase out the LIVC amount differently depending on the torque range available.
In this way, by adjusting an EGR and/or an LIVC schedule responsive to a change in the peak achievable torque, reflective of a change in the fuel composition, the benefits of EGR and/or LIVC may be utilized, as needed, over an extended range of operating torques while addressing fuel efficiency and engine performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.